Several of the alternatives linked in this post’s comments make no effort to encrypt at all; they simply try to con users into sharing files with their intermediate server in plaintext, as if somehow that’s an acceptable thing to do.

If you don’t trust Mozilla or you are sharing information that a nation-state attacker would coerce Mozilla into revealing, then you’re already set up to encrypt the file first yourself - at which point you can send it with any service, include Firefox Send.